1. Field of the Invention
The present invention relates to a method of annealing a core of an amorphous magnetic alloy used for a transformer or the like.
2. Discussion of Background
The use of an amorphous magnetic alloy, which chiefly contain metals (Fe, Co, Ni) and semimetals (B, C, Si, P) and are rapidly quenched in a wound or laminated core of a transformer, has been studied. A plate made of this alloy has an iron loss 1/3 to 1/4 that of a silicon steel plate, which is the conventional core material when no strain is present in the material. This alloy, therefore, has excellent magnetic characteristics.
However, since strain is produced in an amorphous magnetic alloy plate upon quenching, iron loss in this state is large and magnetic characteristics are thus considerably decreased. In order to obtain the original excellent characteristics, the amorphous magnetic alloy is annealed to remove strain after the core made of this alloy is assembled into a core, thus reducing iron loss.
A core consisting of an amorphous magnetic alloy is annealed with an electric heating source. Referring to FIG. 1, a wound core 1 consisting of an amorphous magnetic alloy 2 and a DC magnetic field generating coil 3 wound therearound is placed in a thermostat chamber 4 in which an inert gas is sealed so as to prevent oxidation of the core. Then, temperature of the interior of the chamber 4 is raised to a predetermined value upon flowing a DC current in the coil 3 from a DC power source (or battery) 5. Thus, the core 1 is heated to an annealing temperature and is maintained at this temperature for a predetermined period of time. Thereafter, the heater is turned off, and the core 1 is cooled while applying a DC magnetic field thereto. In this manner, annealing of the core 1 is completed.
The annealing temperature and time vary in accordance with amorphous magnetic alloy materials. For example, in METGLAS2605S2 (chemical composition: Fe.sub.78 B.sub.13 Si.sub.9) available from Allied Corporation and considered the best transformer core material, the annealing temperature is selected to be 390.degree. C. to 410.degree. C. and time is approximately 2 hours. Thus, the annealing temperature range is fairly narrow, i.e., 400.degree. C..+-.10.degree. C. Other amorphous magnetic alloy materials also have a narrow annealing temperature range. However, in the above conventional method, since the core 1 is externally heated by radiation, heat at the outer surface of the core 1 cannot be sufficiently transmitted to the interior thereof with the result that the temperature distribution in the core is nonuniform. Therefore, it is difficult to maintain the temperatures both at the outer surface and in the core within the cited narrow range at the same time. For this reason, thermal stress arising from a temperature difference occurs in the plates 2 of the core 1 and degrades the magnetic characteristics.